ORIGINAL ARTICLE
Synthesis, characterization and biological studies of Co(II), Ni(II), Cu(II) and Zn(II) complexes with
pyrral- L -histidinate
D. Arish, M. Sivasankaran Nair
*Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627 012, India
Received 11 July 2010; accepted 18 August 2010 Available online 22 August 2010
KEYWORDS Schiff base;
IR;
Electronic;
ESR;
Antimicrobial activity
Abstract The Schiff base ligand, pyrral-L-histidinate(L) and its Co(II), Ni(II), Cu(II) and Zn(II) complexes were synthesized and characterized by elemental analysis, mass, molar conductance, IR, electronic, magnetic measurements, EPR, redox properties, thermal studies, XRD and SEM.
Conductance measurements indicate that the above complexes are 1:1 electrolytes. IR data show that the ligand is tridentate and the binding sites are azomethine nitrogen, imidazole nitrogen and carboxylato oxygen atoms. Electronic spectral and magnetic measurements indicate tetrahedral geometry for Co(II) and octahedral geometry for Ni(II) and Cu(II) complexes, respectively. The observed anisotropicgvalues indicate the presence of Cu(II) in a tetragonally distorted octahedral environment. The redox properties of the ligand and its complexes have been investigated by cyclic voltammetry. Thermal decomposition profiles are consistent with the proposed formulations. The powder XRD and SEM studies show that all the complexes are nanocrystalline. Thein vitrobiolog- ical screening effects of the synthesized compounds were tested against the bacterial species,Esch- erichia coli, Bacillus subtilis, Pseudomonas aeruginosaand Staphylococcus aureus; fungal species,
* Corresponding author. Address: Department of Chemistry, Fac- ulty of Science, Manonmaniam Sundaranar University, Tirunelveli 627 012, Tamil Nadu, India. Mobile: +91 9443540046; fax: +91 462 23334363.
E-mail addresses: [email protected] (D. Arish), msnairchem@
rediffmail.com(M.S. Nair).
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doi:10.1016/j.arabjc.2010.08.011
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Aspergillus niger,Aspergillus flavusandCandida albicansby the disc diffusion method. The results indicate that complexes exhibit more activity than the ligand. The nuclease activity of the ligand and its complexes were assayed on CT DNA using gel electrophoresis in the presence and absence of H2O2.
ª2010 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction
Metal ions can simulate some of the features of enzymic active sites by acting as a trap for the Schiff base formed between pyridoxal and amino acids (Casella and Gullotti, 1983; Koh et al., 1996). Transition metal complexes of salicylaldehyde–
amino acid Schiff bases are nonenzymatic models for pyri- doxal–amino acid systems (Boghaei and Gharagozlou, 2007).
Further, the complexes of amino acid Schiff bases possess anti- bacterial, antifungal and antitumor activities (Sakıyan et al., 2004; Sari and Perihan, 2003; Offiong et al., 2000; Wang et al., 2005). The importance of histidyl residue as a metal ion-binding site in biological systems is well recognized (Des- champs et al., 2003). Many of these systems are enzymes and proteins that contain copper and iron as prosthetic groups.
They are mostly involved in oxygen transport, metal storage and transport, electron transfer or oxygen activation (Brill, 1977; Beinert, 1980; Spiro, 1980). In continuation of recent re- ports from this laboratory on some amino acid Schiff base me- tal complex systems (Nair and Raj, 2009; Arish and Nair, 2010; Nair et al., 2010), the present paper describes synthesis, characterization and biological studies of a Schiff base ligand derived from pyrrole-2-carboxaldehyde and L-histidine and its Co(II), Ni(II), Cu(II) and Zn(II) complexes.
2. Experimental 2.1. Materials
Pyrrole-2-carboxaldehyde was obtained from Himedia.L-histi- dine was purchased from Sigma and used without further puri- fication. Calf Thymus (CT) DNA was obtained from Genei, Bangalore. Co(II), Ni(II), Cu(II) and Zn(II) chlorides were Merck samples. All other reagents and solvents were pur- chased from commercial sources and were of analytical grade.
2.2. Synthesis of Schiff base ligand
Pyrrole-2-carboxaldehyde (0.741 g, 5 mmol) in 20 ml of abso- lute MeOH and a stirred methanol solution (20 ml) ofL-histi- dine (0.776 g, 5 mmol) containing KOH (0.28 g, 5 mmol) was added dropwise with stirring and refluxed at 50C for 8 h.
The volume of the resulting solution was reduced to one-third using a rotary evaporator. An air sensitive brownish yellow precipitate was filtered off, washed with anhydrous ether, cold ethanol and then driedin vacuo over anhydrous CaCl2. The Schiff base ligand was recrystallized with 80:20 ethanol and water mixture. The yield was found to be 57%.
2.3. Synthesis of the Schiff base metal(II) complexes
To the brownish yellow solution of Schiff base ligand in 20 ml of hot MeOH (5 mmol), a solution of metal(II) chloride in
20 ml of aqueous MeOH (5 mmol) was added dropwise with constant stirring. The reaction mixture was heated under reflux for 2 h and the volume was reduced to half of the initial vol- ume under reduced pressure. The precipitate was filtered off, washed several times with cold EtOH, ether and then dried in vacuoover anhydrous CaCl2.
2.4. Physical measurements
Elemental analysis was done using a Perkin-Elmer elemental analyzer. Fast Atom Bombardment (FAB) mass spectra were recorded on a JEOL SX 102/DA-6000 mass spectrometer/data system using argon/xenon (6 kV, 10 mA) as the FAB gas. The metal contents in the complexes were determined by standard EDTA titration (Vogel, 1978). Molar conductance of the com- plexes was measured in MeOH (103M) solutions using a cor- onation digital conductivity meter. IR spectra were recorded in KBr discs on a JASCO FT/IR-410 spectrometer in the 4000–
400 cm1 region. The electronic spectra were recorded on a Perkin-Elmer Lambda-25 UV–Vis spectrometer. Room tem- perature magnetic measurements were performed on a Guoy’s balance by making diamagnetic corrections using Pascal’s con- stant. The X-band ESR spectrum of the Cu(II) complex was recorded on a Varian E112 X-band spectrometer. Cyclic vol- tammetric measurements were carried out in a Bio-Analytical System (BAS) model CV-50W electrochemical analyzer. The three electrode cells comprised of a reference Ag/AgCl, auxil- iary platinum and working glassy electrodes. Tetrabutylammo- nium perchlorate was used as supporting electrolyte. Thermal analysis was carried out on a SDT Q 600/V8.3 build 101 ther- mal analyzer with a heating rate of 20C/min using N2atmo- sphere. Powder XRD was recorded on a Rigaku Dmax X-ray diffractometer with Cu-Karadiation (kKa= 1.5406 A˚). SEM images were recorded in a Hitachi SEM analyzer.
2.5. Antimicrobial activities
Antibacterial and antifungal activities of the ligand and its complexes were tested in vitro against the bacterial species Escherichia coli(MSU B05),Bacillus subtilis(MSU B06),Pseu- domonas aeruginosa (MSU B07) and Staphylococcus aureus (MSU B08); fungal species, Aspergillus niger (MSU F04), Aspergillus flavus (MSU F05) and Candida albicans (MSU F06) by the disc diffusion method (Bauer et al., 1996).Amika- cin,Ofloxacin and Ciprofloxacin were used as standards for antibacterial activity and Nystatin was used as standard for antifungal activity. The test organisms were grown on nutrient agar medium in petri plates. The compounds were prepared in DMSO and soaked in a filter paper disc of 5 mm diameter and 1 mm thickness. The discs were placed on the previously seeded plates and incubated at 37C and the diameter of inhi- bition zone around each disc was measured after 24 h for anti- bacterial and 72 h for antifungal activities. The minimum
inhibitory concentration (MIC) was determined by serial dilu- tion technique.
2.6. Gel electrophoresis
A solution of CT DNA in 0.5 mM NaCl/5 mM Tris–HCl (pH 7.0) gave a ratio of UV absorbance at 260 and 280 nm (A260/
A280) of 1.8–1.9, indicating that the DNA was sufficiently free of proteins. Cleavage reactions were run between the metal complexes and DNA, and the solutions were diluted with load- ing dye using 1% agarose gel. Then 3lL of ethidium bromide (0.5lg/ml) was added to the above solution and mixed well.
The warmed agarose was poured and clamped immediately with a comb to form sample wells. The gel was mounted into electrophoretic tank; enough electrophoretic buffers were added to cover the gel to a depth of about 1 mm. The DNA sample (30lM), 50lM metal complex and 500lM H2O2 in 50 mM Tris–HCl buffer (pH 7.1) were mixed with a loading dye and loaded into the well of the submerged gel using a micropipette. The electric current (50 mA) was passed into the running buffer. After 1–2 h the gel was taken out from the buffer. After electrophoresis, the gel was photographed un- der UV transluminator (280 nm) and documented.
3. Results and discussion
The analytical data and physical properties of the Schiff base ligand (L) and its complexes are listed inTable 1. The Schiff base ligand is soluble in common organic solvents such as EtOH, CHCl3 and MeOH. The proposed structure of the Schiff base ligand is given in Fig. 1. The air sensitive Schiff base ligand is not stable for long under ordinary laboratory conditions. It is highly affected by direct sunlight and is decomposed within 1 week. Hence, it was used, as prepared, without delay. However, the resulting metal complexes were found to be stable at room temperature and insoluble in com- mon solvents such as EtOH, CHCl3, MeOH and water but sol- uble in DMF and DMSO. The elemental analysis data (Table 1) confirmed that the complexes have a 1:1 molar ratio between the metal and the Schiff base ligand. The molar con- ductance of all the complexes was measured in DMSO using 103M solutions at room temperature. The conductance (Table 1) values indicate that all the complexes are in 1:1 elec- trolytic nature (Geary, 1971). In addition, the presence of
uncoordinated chloride is also confirmed by the silver nitrate test (Vogel, 1978).
3.1. Mass spectra
The mass spectrum of the Schiff base ligand shows a well-de- fined molecular ion peak at m/z= 270 (relative inten- sity = 16%), which coincides with a formula weight of the Schiff base. The spectrum of the ligand shows a series of peaks atm/z231 (100%), 153 (58%), 107 (72%), 89 (60%) and 56 (53%) corresponding to its various fragments. The mass spec- tra of Co(II), Ni(II), Cu(II) and Zn(II) complexes show a molecular ion peak (M+) atm/z344 (18%), 414 (12%), 420 (23%) and 350 (28%), respectively, suggesting the complex to be monomeric, confirming the stoichiometry of the metal to ligand ratio to be 1:1 in all the complex systems.
3.2. IR spectra
The important IR spectral data of the Schiff base ligand and its complexes are given inTable 2. The IR spectrum of the free Schiff base ligand exhibits a sharp band at 1632 cm1, due to the azomethine group m(C‚N). On complexation this band was shifted to a lower frequency in the1625–1618 cm1range indicating the coordination of the azomethine nitrogen atom to the metal ion. The IR spectrum of the Schiff base ligand exhibits a broad band centered at 1605 cm1due to asymmetric car- boxylmas(COO)-stretching modes, which is probably broad- ened because of an overlap with aromatic ring carbon stretching and imidazole ring imido nitrogen-stretching fre- quency (Pessoa et al., 1998; Nakamoto, 1978). In addition, the band at 1410 cm1 can be ascribed to the symmetric carboxyl ms(COO) group. During complexation they were shifted to lower frequency by5–16 cm1indicating the link- age between the metal ion and carboxylato oxygen atom. The
Table 1 Analytical and physical data of the ligand and its complexes.
Compound Empirical formula Color Elemental analysis Found (calcd) (%)
Molar conductance (X1cm2mol1)
C H N M
L KC11H11N4O2 Brown 48.72
(48.87) 4.23 (4.10)
20.08 (20.73)
– –
[CoL(H2O)]Cl CoC11H13N4O3Cl Violet 38.16 (38.45)
3.97 (3.81)
16.04 (16.30)
17.11 (17.15)
90 [NiL(H2O)3]ClÆ2H2O NiC11H21N4O7Cl Dark green 31.72
(31.80) 5.01 (5.09)
12.97 (13.49)
14.03 (14.13)
82 [CuL(H2O)3]ClÆ2H2O CuC11H21N4O7Cl Green 31.32
(31.43) 4.95 (5.04)
12.92 (13.33)
15.23 (15.12)
97 [ZnL(H2O)]Cl ZnC11H13N4O3Cl Yellow 36.99
(37.14) 3.93 (3.74)
15.97 (16.00)
18.44 (18.68)
95
Figure 1 Proposed structure of the Schiff base ligand.
large difference betweenmas(COO) andms(COO) (200 cm1) indicates the monodentate-binding nature of the carboxylato group in the complexes (Nakamoto, 1978; Deacon and Phillips, 1980). The presence of a water molecule in the complexes is indicated by the very broad absorption band centered at 3330 cm1(Hosny, 1998). In the lower frequency region the weak bands observed at 547–530 and 423–412 cm1have been assigned to the m(M–O) andm(M–N) vibrations, respectively (Nakamoto, 1978; Ouf et al., 2010). Thus the IR spectrum indi- cates that the Schiff base ligand acts as a tridentate N2O donor involving azomethine nitrogen, imidazole nitrogen and car- boxylato oxygen donor atoms.
3.3. Electronic spectra and magnetic measurements
The electronic spectral and magnetic moment data of the com- plexes are given in Table 3. The electronic spectrum of free Schiff base ligand shows a broad band at 367 nm, which is as- signed to the p–p* transition of the azomethine (>C‚N–) chromophore. On complexation this band was shifted to lower wavelengths, suggesting the coordination of azomethine nitro- gen to the central metal ion. The electronic spectrum of tetrahedral Co(II) complexes is reported to have only one absorption band in the visible region due to the
4A2(F)fi4T1(P) transition (Lever, 1984). The spectrum of the present Co(II) complex has only one band in the visible re- gion at 568 nm (Table 1), which indicates tetrahedral geometry for the complex. The electronic spectrum of the Ni(II) complex shows three bands at1100, 720 and 420 nm (Fig. 8), respec- tively, due to the metal-based transitions3A2g (F)fi3T2g (F),
3A2g (F)fi3T1g (F) and3A2g (F)fi3T1g(P), indicating octa- hedral orientation of donor centers around the metal ion (Rosu et al., 2010). The Cu(II) complex exhibits a broad absorption at 800 nm and a weak shoulder at 430 nm. This is due to d–d transitions, which correspond to the2Egfi2A1g
and 2Egfi2B1g transitions, respectively (Nag et al., 2005).
This supports the distorted octahedral structure for the Cu(II) complex.
The Co(II) complex has a magnetic moment of 4.17 BM (Table 1), which is in agreement with the reported value for tet- rahedral Co(II) complexes (Sevagapandian et al., 2000). The present Ni(II) complex shows a magnetic moment value of 3.07 within the range of 2.9–3.3 BM (Sharaby, 2007) suggest- ing octahedral environment. The Cu(II) complex shows a mag- netic moment value of 1.98 BM, which is slightly higher than the spin-only value 1.73 BM expected for the distorted octahe- dral Cu(II) complexes (Mohamed et al., 2009).
3.4. Electrochemical studies
All cyclic voltammograms were recorded in acetonitrile solu- tion at a scan rate of 100 mV s1in the potential range +2.0 to 2.0 V. The Schiff base ligand and Zn(II) complex have not shown any peaks in this potential range under similar con- ditions. The cyclic voltammogram of the Co(II) complex shows a well-defined redox process corresponding to the for- mation of the quasi-reversible Co(II)/Co(I) couple. The catho- dic peak at 0.848 V versus Ag/AgCl and the associated anodic peak at0.640 V correspond to the Co(II)/Co(I) cou- ple. The peak to peak separation (DEp= 0.208 V) indicates a quasi-reversible one-electron transfer process. The redox property of the Ni(II) complex displayed an anodic and asso- ciated cathodic peaks at 0.898 and 0.506 V, respectively, corresponding to the formation of the quasi-reversible (DEp= 392) one-electron reduction Ni(II)/Ni(I) couple. The Cu(II) complex displayed a cathodic peak at0.868 V versus Ag/AgCl with the corresponding anodic wave at 0.663 V on the reverse scan (Fig. 2). The peak separation value (DEp= 205 V) indicates a totally quasi-reversible character for the one-electron transfer reaction of metal-based Cu(II)/
Cu(I) couple.
3.5. ESR studies
The ESR spectra of the Cu(II) complex (Fig. 3) in a polycrys- talline state at room temperature exhibited anisotropic signals withgvaluesgk= 2.13 andg^= 2.03, respectively, which is characteristic for the axial symmetry of Cu(II) complexes (Chandra and Gupta, 2004; Rajalakshmi et al., 2008). Thegk and g^ values are closer to 2 and gk>g^. This suggests a tetragonal distortion around Cu(II) corresponding to elonga- tion along the four-fold symmetry Z-axis (Mohamed et al., 2009). The trendgk(2.13) >g^(2.03) >ge(2.0023) shows that Table 2 IR spectral data of the ligand and its complexes (cm1).
Compound amazo.(C‚N) mas(COO)
bmimida ring(C‚N)
ms(COO) cm(H2O) m(M–O) m(M–N)
L 1632 1605 1410 – – –
[CoL(H2O)]Cl 1624 1592 1398 3325 530 412
[NiL(H2O)3]ClÆ2H2O 1622 1595 1398 3330 545 423
[CuL(H2O)3]ClÆ2H2O 1618 1595 1397 3325 540 420
[ZnL(H2O)]Cl 1625 1597 1400 3320 547 415
a Azomethine stretching frequency.
b Imidazole ring imido nitrogen stretching frequency.
c Water molecule stretching frequency.
Table 3 Electronic and magnetic moment data of metal complexes.
Compound Electronic
spectra wavelength (nm)
Magnetic moment (BM)
Geometry
L 220, 250, 367 – –
[CoL(H2O)]Cl 568 4.17 Tetrahedral
[NiL(H2O)3]ClÆ2H2O 420, 720, 1100 3.07 Octahedral [CuL(H2O)3]ClÆ2H2O 430, 800 1.98 Octahedral
the unpaired electron is localized in the dx2y2orbital of the Cu(II) ions in the complex (Krishna et al., 2008). In addition, an exchange coupling interaction between two Cu(II) ions is explained by Hathaway expression G= (gk2)/(g^2) (Hathaway and Tomlinson, 1970). If the valueG> 4.0, the ex-
change interaction is negligible and ifG< 4.0, a considerable exchange coupling is present in the solid complex. In the pres- ent Cu(II) complex, the ‘G’ value (4.33) is >4 indicating that there is no interaction in the complex. Moreover, the absence of a half field signal at 1600 G corresponding to DM= ±2 transitions indicates the absence of any Cu–Cu interaction in the complex (Gaballa et al., 2007). Kivelson and Neiman (1961)have shown that for an ionic environmentgkis 2.3 or larger, but for a covalent environment gk is less than 2.3.
Thegkvalue for the present complex is 2.13, indicating a sig- nificant degree of covalency in the metal–ligand bond.
3.6. Thermogravimetric studies
Thermogravimetric studies have been made in the temperature range 25–900C. The thermal stability data of the complexes are listed in Table 4. Thermal decomposition curves of the Co(II) and Zn(II) complexes show a similar sequence of two decomposition steps. In the first step, Co(II) and Zn(II) com- plexes lost one molecule of coordinated water and a chloride ion as HCl gas between 155 and 270C, which is an endother- mic process as the DTA curve indicates. The next exothermic step of the thermal degradation that occurs between 360 and 615C corresponds to the removal of organic ligand moiety leaving a metal oxide residue. The thermal decomposition of the Ni(II) and Cu(II) complexes occurs in three stages. The first thermal dehydration begins at 50C and ends at 140C with an endothermic peak. The observed mass loss is attrib- uted to the decomposition of two lattice water molecules.
The second endothermic step of the thermal degradation that occurs between 156 and 310C corresponds to the decomposi- tion of the three coordinated water molecules and a chloride ion as HCl gas. Above this temperature a gradual weight loss continues up to 630C, which corresponds to the decomposi- tion of the Schiff base ligand from the metal chelate. The hor- izontal thermal curve observed above 630C corresponds to a metal oxide residue. The thermal analysis curves of the Cu(II) complex are given inFig. 4. Based on the above results, the proposed structure of the complexes is given inFig. 5.
Figure 2 Cyclic voltammogram of the [CuL(H2O)3]ClÆ2H2O complex.
Figure 3 ESR spectrum of the [CuL(H2O)3]ClÆ2H2O complex.
Table 4 Thermogravimetric data of Co(II), Ni(II), Cu(II) and Zn(II) complexes.
Complex Temperature (C) DTA peak (C) % weight loss Obs. (calcd)
Process
[CoL(H2O)]Cl 155–270 253 endo 15.70 (15.54) Loss of coordinated water molecule and chloride ion 290 endo
380–610 420 exo 62.21 (62.50) Loss of organic moieties
>610 548 exo Residue 22.09 (21.80) CoO
[NiL(H2O)3]ClÆ2H2O 60–130 125 endo 8.48 (8.69) Loss of lattice water molecule
156–310 267 endo 21.79 (22.61) Loss of coordinated water molecule and chloride ion 325–645 405 exo 51.31 (51.73) Loss of organic moieties
>645 570 exo Residue 18.42 (17.87) NiO
[CuL(H2O)3]ClÆ2H2O 52–140 120 endo 8.89 (9.28) Loss of lattice water molecule
160–307 265 endo 21.26 (21.30) Loss of coordinated water molecule and chloride ion 330–630 400 exo 50.73 (51.19) Loss of organic moieties
>630 575 exo Residue 19.12 (18.80) CuO
[ZnL(H2O)]Cl 160–270 255 endo 15.44 (15.28) Loss of coordinated water molecule and chloride ion 291 endo
368–615 420 exo 61.54 (61.42) Loss of organic moieties
>615 555 exo Residue 23.02 (22.85) ZnO
3.7. Powder XRD and SEM
The powder XRD patterns of Co(II), Ni(II), Cu(II) and Zn(II) complexes were recorded over the 2h= 0–80 A˚ range and are shown inFig. 6. From the data, all the complexes show sharp peaks indicating their crystalline nature. The average crystal- lite sizes of the complexesdXRDwere calculated using Scher- rer’s formula (Dhanaraj and Nair, 2009). The Co(II), Ni(II), Cu(II) and Zn(II) complexes have an average crystallite size of 89, 56, 16 and 39 nm, respectively, suggesting that the com- plexes are in a nanocrystalline phase.
The SEM micrographs of the Co(II), Ni(II), Cu(II) and Zn(II) complexes are given in Fig. 7. The SEM pictures of the complexes show different morphological structures and the presence of small grains in the nanometer range. The
Co(II) complex displays closely packed cauliflower-like mor- phological structures, while Ni(II) complex has a broken-stone like surface morphology. The SEM picture of the Cu(II) com- plex exhibits clusters of cotton like morphology and the pres- ence of small grains are uniformly arranged. The Zn(II) complex has small ice bar like morphology. The average grain size found from SEM shows that the complexes are polycrys- talline with nano sized grains.
3.8. Antimicrobial activity
The results of the antimicrobial activities are summarized in Table 5. The standard error for the experiment is ±0.001 cm and the experiment is repeated three times under similar con- ditions. DMSO is used as negative control and Amikacin, Ofloxacin andCiprofloxacin were used as positive standards for antibacterial and Nystatin for antifungal activities. The results show that the metal chelates are more active than the starting materials and the free ligands. The antimicrobial Figure 4 Thermal analyses curves of the [CuL(H2O)3]ClÆ2H2O
complex.
Figure 5 Proposed structure of the metal complexes.
Figure 6 XRD patterns of (a) [CoL(H2O)]Cl, (b) [NiL(H2O)3]ClÆ2H2O, (c) [CuL(H2O)3]ClÆ2H2O, (d) [ZnL(H2O)]Cl complexes.
activity of the complexes is often greater than those of the cor- responding free ligands, explained on the basis of Overtone’s concept and chelation theory (Priya et al., 2009). The Schiff base ligand has good activity against the bacterial strain E. coli whereas all the complexes exhibited excellent in vitro antibacterial activity against Gram-positive and Gram-nega- tive organisms. Among the complexes, Ni(II) and Cu(II) com- plexes were highly active against both Gram-positive and Gram-negative organisms and this antibacterial effect is equal to that of some antibiotics. The Co(II) complex was found to be highly active towardsE. coli.
The fungicidal screening shows thatC. albicanswas inhib- ited by Co(II), Ni(II) and Cu(II) complexes having MIC values 15, 15 and 08, respectively. The free Schiff base ligand was
more active than some of its metal complexes against the fungi A. niger. The Ni(II) and Cu(II) complexes show good activity against all the tested fungal strains (MIC < 100). The activity order of the synthesized compounds is as follows:
Cu(II) > Ni(II) > Co(II) > Zn(II) > L.
3.9. Nuclease activity
Gel electrophoresis experiments using CT DNA were per- formed with the ligand and its complexes in the presence and absence of H2O2 as an oxidant. The results (Fig. 8) indicate that all the complexes can interact with CT DNA in the pres- ence of H2O2. However, Ni(II) and Cu(II) complexes can cleave DNA effectively compared to other systems. Probably Figure 7 SEM images of (a) [CoL(H2O)]Cl, (b) [NiL(H2O)3]ClÆ2H2O, (c) [CuL(H2O)3]ClÆ2H2O, (d) [ZnL(H2O)]Cl complexes.
Table 5 In vitroantimicrobial activity (MIC,lg/ml) of the compounds and standard reagents.
Compound Bacterial species Fungal species
E. coli B. subtilis P. aereuguinosa S. aureus A. niger A. flavus C. albicans
L 20 80 75 >100 25 >100 80
[CoL(H2O)]Cl 18 75 20 50 35 26 15
[NiL(H2O)3]ClÆ2H2O 15 14 10 25 34 15 15
[CuL(H2O)3]ClÆ2H2O 05 18 32 28 12 25 08
[ZnL(H2O)]Cl >100 35 24 30 22 75 75
Amikascina 05 05 04 05 – – –
Ciprofloxacina 05 05 05 05 – – –
Ofloxacina 10 02.5 02.5 05 – – –
Nystatina – – – – 06 06 05
a Standards.
this is due to the formation of the redox couple of the metal ions and its behavior. The redox cycling between M(II) and M(I) can catalyze the production of highly reactive hydroxyl radicals. These hydroxyl radicals participate in the oxidation of the deoxyribose moiety, followed by the hydrolytic cleavage of the sugar-phosphate backbone. It is observed that most cleavage cases are caused by the metal ions reacting with H2O2 to produce diffusible hydroxyl radicals or molecular oxygen, which may damage DNA through a Fenton type chemistry (Babu et al., 2007). The presence of a smear in the gel diagram indicates the presence of a radical cleavage. In addition, the nuclease activity of the complexes has also been investigated in the absence of the oxidant H2O2. Moreover, no effect was observed indicating the involvement of hydroxyl radicals in the cleavage process.
4. Conclusion
Co(II), Ni(II), Cu(II) and Zn(II) complexes with N2O donor Schiff base ligand derived from pyrrole-2-carboxaldehyde andL-histidine were synthesized. A comparative study of their physico-chemical properties have been made through elemen- tal analysis, molar conductance, mass, IR, electronic spectra, magnetic moment, ESR, CV, TG/DTA, powder XRD and SEM. The conductance data indicate that all the complexes are 1:1 electrolytes. The IR data reveal that the Schiff base li- gand pyrral-L-histidinate coordinates through the azomethine nitrogen, imidazole nitrogen and carboxylato oxygen atoms.
A tetrahedral geometry for Co(II) and Zn(II) complexes and octahedral/distorted octahedral geometry were assigned for Ni(II) and Cu(II) complexes. The ESR spectrum of the Cu(II) complex reveals its tetragonally distorted octahedral geometry.
The thermal decomposition of the Co(II) and Zn(II) complexes is a two-stage process while the Ni(II) and Cu(II) complexes displayed a three-step process. Powder XRD results show that the complexes are of crystalline nature. The SEM image of Co(II) complex exhibits cauliflower-like morphological struc- tures. Ni(II) and Cu(II) complexes were found to be more ac- tive against most of the microbes and they effectively cleaved CT DNA in the presence of an oxidant.
Acknowledgment
One of the authors (D. Arish) is grateful to National Testing Service, India, CIIL, Mysore for providing the fellowship.
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Figure 8 Gel electrophoresis diagram for the reaction done in the control experiment with an incubation time of 60 min at 30C in dark condition: lane 1: DNA + H2O2; lane 2: DNA + ligand + H2O2; lane 3: DNA + [CoL(H2O)]Cl + H2O2; lane 4:
DNA + [NiL(H2O)3]ClÆ2H2O; lane 5: DNA + [CuL(H2O)3]ClÆ2- H2O + H2O2; lane 6: DNA + [ZnL(H2O)]Cl + H2O2.
